A three-way lamp, e.g., three way incandescent lamp, produces three levels of light intensity (i.e., low, medium, and high), typically using two lamp filaments within the same optical housing. The two filaments are typically of different wattages. For example, one lamp filament can be a low wattage filament, and the other filament can be a high wattage filament.
Conventionally, these two filaments are connected in parallel to the lamp base. The lamp base itself has two contacts and a neutral contact. Each of the filaments operates at full voltage when activated.
Proper installation of the three-way lamp is achieved by using a three-way lamp socket, which has three contacts instead of the usual two for a single filament lamp. This third contact is typically off center in the bottom of the socket, and makes contact with the second filament circuit.
The three-way lamp is controlled using a three-way switch, which itself has four positions. Starting from the ‘off’ position, the switch can sequentially connect power to one filament (typically the lower wattage filament,), then the other filament, and then both filaments.